Composite refractory materials

Abstract

A relatively dense composite refractory material which has reasonable strength and good thermal shock resistance comprises an alumina matrix material within which are dispersed particles of polycrystalline zirconia comprises from 1.0 to 40 percent by volume of the composite material. Each particle of zirconia is an agglomerate of microcrystals which are strongly bonded together. Due to the agglomerates of the bonding together of the microcrystals at their grain boundaries. The microcrystals contain no matrix material within their agglomerates, and exhibit strong thermal expansion anisotropy. They have a size such that cracks do not form spontaneously within the agglomerates during cooling from about 1600° C. to room temperature. Typically the agglomerates have a mean diameter of from 10 to 15 micrometers, while the average diameter of the microcrystals in an agglomerate is from 1 to 2 micrometers.

I claim: 
     
       1. A composite alumina/zirconia refractory material comprising an alumina matrix with particles of zirconia dispersed therein, said composite material having been formed by die pressing a mixture of alumina powder and the zirconia particles, followed by sintering in the absence of applied pressure at a temperature of about 1600° C., said composite material having a porosity which does not exceed 12 percent, wherein the particles of zirconia (a) occupy from 1.0 percent by volume to 40 percent by volume of the composite refractory material;   (b) each comprise a polycrystalline agglomerate of microcrystals with no matrix material within the agglomerate, the microcrystals (i) being strongly bonded together, (ii) exhibiting a strong thermal expansion anisotropy, and (iii) having a size such that cracks do not form spontaneously within the agglomerate during cooling from a temperature of about 1600° C. to room temperature; and   (c) each have a mean diameter of from 10 to 15 micrometers, with the microcrystals therein having a mean diameter of from 1 to 2 micrometers.   
     
     
       2. A composite refractory material as defined in claim 1, in which the dispersed zirconia particles comprise from 3 per cent to 10 percent by volume of the composite refractory material. 
     
     
       3. A composite refractory material as defined in claim 1, in which the dispersed zirconia particles comprise about 5.6 percent by volume of the composite refractory material. 
     
     
       4. A composite refractory material as defined in claim 1, in which the alumina matrix phase has a mean particle size in the range from 0.1 micrometer to 5.0 micrometers. 
     
     
       5. A composite refractory material as defined in claim 4, in which the alumina matrix phase has a mean particle size of about 1.0 micrometer. 
     
     
       6. A composite refractory material as defined in claim 1, in which the alumina matrix material contains an insignificant amount of low melting point glassy phases. 
     
     
       7. A composite refractory material as defined in claim 2, in which the dispersed polycrystalline zirconia particles comprise about 10 percent by volume of the composite material. 
     
     
       8. A method of making a composite refractory material as defined in claim 1, said method comprising the steps of (a) mixing together a powder of the alumina material, a powder of the polycrystalline to be dispersed within the matrix material, and a fugitive binder;   (b) allowing the mixture to dry;   (c) granulating the dried powder mixture;   (d) preforming the granulated powder into billets by die pressing;   (e) isostatically pressing the die pressed billets to form green billets for sintering;   (f) heating the green billets at a rate until a sintering temperature of from 1200° C. to 1800° C. is reached;   (g) holding the green billets at the sintering temperature for a period of from 0 to 5 hours; and   (h) furnace cooling the sintered billets to ambient temperature.   
     
     
       9. A method as defined in claim 8, in which the period of hold at the sintering temperature is about 1 hour. 
     
     
       10. A method as defined in claim 3, in which the heating rate is 100° C. per hour, the sintering temperature is 1600° C., and the period of hold at the sintering temperature is one hour.